As a web developer you can build small websites. Mobile app developers can build apps. Most C++ work is part of something bigger, so you're kind of stuck with a job.
If it’s about side money, it usually also implies a smaller, side project, not necessarily, but eventually for smaller businesses to whom technological choices don’t matter.
You just define the outcome together with your client, and use C++ if it’s a technological fit.
There are lots of consulting gigs, or part-time temp positions.
You can also put yourself on sites like upwork, but you will compete with many other people in a race to the bottom, regardless of the language you choose.
You just define the outcome together with your client, and use C++ if it’s a technological fit.